In classical conditioning, a neutral stimulus is paired with an unconditioned stimulus (US), which causes an unconditioned response (UR). The neutral stimulus becomes a conditioned stimulus (CS) and causes a conditioned response (CR).

5 During the acquisition phase of classical conditioning, a neutral stimulus is repeatedly paired with an unconditioned stimulus. As you may recall, an unconditioned stimulus is something that naturally and automatically triggers a response without any learning.

Tara Moore / Getty Images. In the learning process known as classical conditioning, the unconditioned stimulus (UCS) is one that unconditionally, naturally, and automatically triggers a response. In other words, the response takes place without any prior learning.

The unconditioned stimulus is usually a biologically significant stimulus such as food or pain that elicits an unconditioned response (UR) from the start. The conditioned stimulus is usually neutral and produces no particular response at first, but after conditioning it elicits the conditioned response.

Simply put, a conditioned stimulus makes an organism react to something because it is associated with something else. For example, Pavlov’s dog learned to salivate at the sound of a bell. In classical conditioning, the conditioned response (CR) is the learned response to the previously neutral stimulus.

An UCS can trigger a response naturally. This response is a biological reaction. A person or animal usually does not have control over this behavior​2​. Here are some examples of unconditioned stimulus.